1. Field of the Invention
The present invention relates to an improved foam and method of producing such foam for a variety of applications, including semiconductor polishing pads and belts.
2. Description of the Related Art
Silicon wafers are produced as precursors from which microelectronic semiconductor components are produced. The wafers are sliced or cut from cylindrical silicon crystals, parallel to their major surfaces, to produce thin disks, typically 20-30 centimeters in diameter, although larger or smaller wafers are possible. The resulting wafers must be polished to give flat and planar surfaces for proper formation of electronic components to form integrated chip semiconductor devices. Typically, a 20-cm diameter wafer will produce 100 or more microprocessor chips.
The design size of such integrated chips is steadily decreasing, while the number of layers applied, e.g., by various sequences of depositing, patterning, and etching of features onto the silicon surface, is rising. Present semiconductors typically incorporate up to 8 or more metal layers, and it is expected that future designs will contain even more layers. The decrease in the size of the circuitry and the increase in the number of layers applied are leading to even more stringent requirements on the smoothness and planarity of the silicon and semiconductor wafers throughout the chip manufacturing process, since uneven surfaces may undermine the patterning process and the general integrity of the resulting circuit.
In the semiconductor chip fabrication process, it is well-known that there is a need to polish a semiconductor wafer. This polishing is typically-accomplished by a chemical mechanical process (CMP). One standard CMP wafer polishing technique is to position a wafer over a rotating polishing pad that is usually disk-shaped, and is mounted on a large turntable. A chemical-mechanical polishing slurry is usually applied to the surface of the pad, and the wafer is held in place by an overhead wafer carrier while being polished by the rotating pad and slurry. The slurry is generally made up of an aqueous solution with metallic or non-metallic particles such as, for example, aluminum or silica abrasives that create the added friction for the polishing process.
A significantly different approach is the so-called Linear Planarization Technology (LPT), wherein the polishing pad is mounted onto a supporting belt and is used to polish the wafer, in place of the flat turntable form of the polishing tool. The belt used in this method is described in EP-A-0696495 and comprises an endless sheet of steel or other high strength material, having a conventional flat polyurethane polishing pad affixed to it with adhesive. As with the rotating pad, the pad used for LPT CMP polishing receives a chemical-mechanical polishing slurry that is distributed over the surface of the belt.
State of the art semiconductor polishing pads are made from high density polyurethane foams that have a functional porous structure, which aids the distribution of the chemical-mechanical polishing slurry and reduces hydroplaning, for example. Such pads are formed from a polymeric composition that comprise a dispersion of thin-walled, hollow plastic beads or “microspheres,” which can potentially provide a controlled and consistent microcellular structure.
However, there are some limitations to the use of hollow microspheres in polishing pads. The size and shape of the foam's cells are restricted to the limited sizes and shapes of the commercially available microspheres. In addition, microspheres may be too abrasive for some delicate polishing operations, e.g. certain steps in semiconductor manufacturing including, but not limited to, chemical mechanical polishing of soft metal layers. Typically, the microspheres are extremely light weight and flammable, posing significant material handling difficulties, including dust explosion hazards. The lightweight microspheres are also difficult to disperse in the polyurethane resins. They tend to clump and foul process equipment, and often entrain significant amounts of air, which leads to problematic variations in porosity of the cured foam. Also, the microspheres can distort, collapse, or melt if processed at high temperatures that are routinely used in processing polyurethanes and other potential pad materials.
Foam density, a measurement of the mass of froth per unit volume, is one of the most important properties of froth, directly affecting the durability and support of the foam. It is commonly measured and expressed in pounds per cubic foot (pcf) or kilograms per cubic meter (kg/m3), but may also be stated as g/cc. Foam density is directly related to the specific gravity of the foam.
Although there are several conventional ways to create high density polyurethane foam, including mechanical frothing and chemical blowing processes, pads produced by the conventional methods have not been successful in semiconductor polishing. While the polishing pads produced by the conventional method may be suitable for polishing glass and other low technology applications, they have not been as successful in semiconductor polishing, which is a more precise and more delicate application, because of the variability in pad cell structure and pad properties. Often times, the density of the foam produced by these conventional frothing methods varies greatly due to the conventional methods' inability to consistently produce foam within a preferable range of specific gravities or because of impurities, e.g. oxygen, contained within the foam. Another significant drawback of the prior art frothing methods is the time involved in producing the froth. Consequently, there is a need for a time efficient method of manufacturing froth falling within an optimal specific gravity range.